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Old 30-06-2006, 12:33   #16
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DC to DC converters

Just thought that you might be interested to know that such products are most properly designed with DC-to-DC converters which, in our case, would accept at least 8 to 18 Volts input and deliver just what the LED array is optimum at delivering the most lumens per Watt input.

I've designed with such little converters for years and they can exceed 97% efficiency. Current limiting resistors generate a total waste of energy and you can figure it out for yourself; the voltage across the limiting resistor divided by the voltage across the LED array is the percent total loss of energy in this case. Add to that factor the lack of operating the LED at an optimum current for a maximum lumen output and you have another loss. Add to that a factor which is the heat loss of each LED.

The total losses for the white LED systems are more than what you will get using a compact flourescent of good quality (you are replacing crap quality flourescent units with crap effeciency LED operation) and waaaay more than what you would get using cold-cathode flourescent assemblies. Keep in mind that all of this is in terms of how many lumens you get per Watt of input power. I know that this doesn't seem intuitive or obvious yet it is true. Yes, it will be some time before the cold-cathode flourescent units become ubiquitous yet it will happen relatively soon.

Keep in mind also that compact flourescent bulbs can be found with just the right "color" temperature for your personal preference from very white to very yellow (I hate the yellow yet am told by the experts that women tend to prefer them).

Don't get me wrong, I love LEDs (especially for little flashlights) I merely am practical about the reality of the physics amongst the various types. HID (high intensity discharge) assemblies are also more efficient yet are too expensive so far to find their way into sub-50 Watt assemblies.

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Old 01-07-2006, 08:05   #17
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Nibblin' on sponge cake,
watchin' the sun bake;
All of those tourists covered with oil.

Oh, sounds good...

Rick, you are right, DC to DC converters...I have a tendency to call all black boxes with wires going in and out Power Supplies (well, not ALL black boxes)..must be old age...

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Old 01-07-2006, 10:34   #18
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Thanks DeepFrz for pointing out my little blooper on my question to Mark about the price.

My job has made my eyes acting funny. I am in the process of finding some bi-focals. Cause right now I'm straining the hell out of my eyes. And I work around alot electronic circuit boards. And they can strain your eyes alot!!

Sorry about asking that question Mark!!

"Those who desire to give up Freedom in order to gain security, will not have, nor do they deserve, either one." - Benjamin Franklin
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Old 06-07-2006, 08:31   #19
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OK - now for the results of Phase II.

I built a 48 LED series-parallel array in order to evaluate using 5mm LEDs vice buying the light bars. The cost breakdown for this array is:

48ea 5mm white LED 10,000mcd = $5.50 (eBay)
12ea 33 ohm carbon 1/2 watt 5% resistors = $3.60 (Jameco electronics)
2ea 2.5in X 6in circuit boards = $8.00 (Radio Shack)
4ea PCB wire terminals = $3.00 (Radio Shack)
8ea Screws / Nuts / Stand-offs = $3.00 (Ace Hardware)
Wire = $0.20 (from spares)
Light fixture sheet metal = $5.50 (local A/C shop)

Total Cost = $28.80

So far so good - this approach is cheaper.

FYI the schematic is simple:
B+ -> LED1 -> LED2 -> LED3 -> LED4 -> Resistor -> GND
For the array I have 12 of these branch circuits in parallel. Measured current draw @12V is 121ma.

So how does it look? I'll try to post the pictures here:

Array itself

Comparison of both units (uncovered)

48 LED Array (Covered)

Light Bars (Covered)

I think the Light Bar unit looks best. The light color is a bit warmer since it uses the SuperFlux LEDs vice the 5mm units. The light angle is much wider than the 48 LED array, due to the different type of LED. It also looks more like the flourescent fixture I'm hoping to replace.

I've also found a cheap ($3.00) current-limiting DC-DC converter that I've ordered and will evaluate.

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Old 07-07-2006, 18:03   #20
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I think you are right, the light strips do look better, at least in the pictures. They have a much nicer hue to my eye. Could you not find LEDs that more closely matched the ones in the light strip.

Many of the "bulb replacement" led arrays use RGB leds together. Then of course you have to figure out how many of each, and then each color uses a different current, etc. However there has been lots of research posted on the web about just that approach. Looking forward to your results with the current regulators. I noticed that bebi electronics use 2 current regulators in thier nav lights.

Good work. I did some research into led lighting a few years ago but have forgotten anything I might have learned...hmmm, maybe I didn't learn anything.

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Old 07-07-2006, 18:39   #21

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FWIW, "White" LEDs are made from several different technologies, each with different color qualities. Then to make things worse, if you really want to buy a couple of dozen of the same color in the same brightness, you have to PAY EXTRA and order them by the color graded lot from the manufacturer. You pay once for color lot, again for brightness lot, and you pay very dearly if you want uniform LEDs in less than 1-10,000 pieces because you'll have to pay a third time to get them free from whoever ordered the sorted lot. Surplus stuff just isn't prime. usually has cold cathode strips and other alternatives available as well. They usually have the Cree-X high brightness LEDs (1 watt or 5 watt) available as well, no guarantee on what grade group they're from but that's one way to use one LED instead of many. Although it still will be a tight (not wide) angle of light.

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